Considerable research has been conducted in the field of measuring and understanding the development of blade tip Vortices trailed into the wakes of rotors. The research and resulting studies have been motivated by the fact that the structure of the tip vortices defines the majority of the induced velocity field surrounding the rotorcraft. The strong, concentrated tip vortices generated by a helicopter rotor blade (or by the proprotors on a tilt rotor) can also be a source of adverse aerodynamic problems, such as blade-vortex interactions (BVI) and vortex-airframe interactions. In each case, it is the high induced velocities surrounding the tip vortices that become a source of unsteady aerodynamic forces, which can be a significant source of rotor noise and airframe vibrations.
In particular, it is known that small changes in the structure of the tip vortices and their positions relative to the rotor blades can have substantial effects on BVI noise. The reduction of rotor noise has become an important goal in the design of new rotorcraft for both military and civil uses.
In principle, it is plausible to modify the structure of the tip vortices by diffusing their concentrated vorticity which can significantly reduce or even eliminate the aforementioned adverse aerodynamic problems. However, the goal of producing the rapid and effective diffusion of vorticity inside tip vortices is not a new approach nor is it an easy one to implement. Various approaches have been considered, such as with the use of various types of tip shape modifications including sub-wings or spoilers. Active flow control and passive flow control have also been suggested for this purpose.
U.S. Pat. No. 6,283,406 relates to the reduction of noise caused by the movement of aircraft rotor blades whereby the rotor blade system includes a number of jets at different locations and orientations at the tip of each blade through which flow is adjusted. In this approach, the tip vortices are spread out and decay rapidly with increasing distance from the blade tip. The decay and spreading out of the vortices reduce the generation of BVI noise when the vortices encounter the following blade. Such an approach recognizes that the jet orientation and flow velocity needed to reduce BVI noise depends on helicopter operating conditions. Consequently, the system provides for adjustment of the jet orientation and flow velocity based on the observed BVI noise reduction.
A computer on board the helicopter monitors the change in BVI noise using one or both of the noise reduction performance monitoring sensors (microphones and/or pressure sensors) and selects jet location and orientation (i.e., turn on a particular jet) and flow rate so as to reduce BVI noise. BVI noise is additionally reduced by providing blade tip air flow injection in which a plurality of air openings are formed in the outboard tip of the rotor blade and in the surface of the rotor blade proximate the tip for expelling pressurized air. In this technique the system of monitored sensors, as well as an additional source of pressurized air for jet flow injection (for example, an air pump) are employed which greatly complicates the electronic and mechanical structures of the system.
The above techniques basically act to modify the tip vortex structure in some way or perhaps change its stability characteristics. However, the reduction of the induced velocity field surrounding the tip vortex has been found difficult to accomplish without incurring some other form of rotor performance penalty which usually appears as an increase in profile power at the rotor. Actively controlled devices also require some power to establish the blowing/suction or unsteady excitation of the boundary layers at the blade tip and further requires additional non-structural mass for the flow control mechanisms.
Therefore, a technique using some form of simple, lightweight, low-cost passive flow control device which incurs little or no adverse effects on overall rotor performance is still needed in the art.